Conventionally, in micro-wave band and millimeter-wave band integrated circuits (hereinafter, referred to as an IC), a coupling line having a length which is one-fourth (1/4) as long as a wavelength of its design center frequency band is employed in a dc blocking capacitor used in interstage coupling because the insertion loss of the coupling line is small. The coupling line is sometimes referred to as edge coupled transmission lines and includes two closely spaced transmission line lengths disposed side-by-side so that electromagnetic waves are transferred, i.e., coupled, from one transmission line to the other along the respective edges of the transmission lines. FIG. 8 is a plan view showing first and second amplification stages in a 90 GHz band amplifier IC as an example. In FIG. 8, reference numeral 1 designates a field effect transistor (hereinafter, referred to as an FET) at the first amplification stage, numeral 2 designates an output side impedance matching circuit at the first amplification stage, numeral 3 designates the above-described coupling line with the 1/4 wavelength (hereinafter, referred to as 1/4 wavelength coupling line), numeral 4 designates an input side impedance matching circuit at the second amplification stage, and numeral 5 designates an FET at the second amplification stage. In addition, reference characters G, S and D designate a gate, a source, a drain, respectively. However, a dc bias circuit for driving the FET, an input side impedance matching circuit at the first amplification stage and an output side impedance matching circuit at the second amplification stage are omitted in FIG. 8. The above-described impedance matching circuit and the coupling line are formed of a metal film on a substrate. The length L.sub.c of the coupling line is equal to .lambda./4 where .lambda. is a wavelength of the transmission line at the design center frequency.
In this case, the I/O impedance of the 1/4 wavelength coupling line is normally 50.OMEGA.. In order to perform the above impedance matching, the I/O impedance at each amplification stage is 50.OMEGA. at the edge of the impedance matching circuit. Generally, as the frequency becomes high, the impedance matching circuit and the 1/4 wavelength coupling line can be miniaturized. However, a length L.sub.om of the output side impedance matching circuit at the first amplification stage is approximately 350 .mu.m and the length L.sub.c of the 1/4 wavelength coupling line is approximately 300 .mu.m in the above example of a 90 GHz band IC, so that the impedance matching circuit and the coupling line occupy a large area in the millimeter-wave IC. Consequently, the chip size can not be significantly reduced.
In the micro-wave and millimeter-wave IC, it is desired that mass productivity be improved and manufacturing costs be reduced by reducing the chip size. However, as above-described, since the prior art impedance matching circuit and the coupling line whose lengths are decided mostly by the frequency occupy a large area, it is difficult to reduce the chip size of the IC.